refactor(api): extract 8 content domains into api/*.ts slices (R7.5 S1)

Move feed, channels, sync, playlists, tags, notifications, saved-views and
quota out of the lib/api.ts god object into per-domain slice modules, each
owning its own types + method slice and spread into the façade. The `api`
object shape and all 54 call sites are unchanged; account/auth/setup/admin
stay inline until S2.

Reunites the interleaved tags read/CRUD that motivated the epic.

frontend/src/lib/api/notifications.ts

@@ -0,0 +1,28 @@
+// The durable, server-backed notification inbox (distinct from the client-side transient
+// toast/bell, which lives only in localStorage). Only touches core's req.
+import { req } from "./core";
+
+// A durable, server-backed notification (the inbox center). Distinct from the client-side
+// transient toast/bell, which lives only in localStorage.
+export interface AppNotification {
+  id: number;
+  type: string;
+  title: string;
+  body: string | null;
+  data: Record<string, unknown> | null;
+  read: boolean;
+  dismissed: boolean;
+  created_at: string | null;
+}
+
+export const notificationsApi = {
+  notifications: (includeDismissed = false): Promise<{ items: AppNotification[]; total: number }> =>
+    req(`/api/me/notifications?include_dismissed=${includeDismissed}`),
+  notificationUnreadCount: (): Promise<{ count: number }> =>
+    req("/api/me/notifications/unread_count"),
+  markNotificationRead: (id: number) => req(`/api/me/notifications/${id}/read`, { method: "POST" }),
+  markAllNotificationsRead: () => req("/api/me/notifications/read_all", { method: "POST" }),
+  dismissNotification: (id: number) =>
+    req(`/api/me/notifications/${id}/dismiss`, { method: "POST" }),
+  clearNotifications: () => req("/api/me/notifications/clear", { method: "POST" }),
+};